Mycosporines and mycosporine-like amino acids: UV protectants or multipurpose secondary metabolites? (original) (raw)

Abstract

Mycosporines and mycosporine-like amino acids (MAAs) are low-molecular-weight water-soluble molecules absorbing UV radiation in the wavelength range 310–365 nm. They are accumulated by a wide range of microorganisms, prokaryotic (cyanobacteria) as well as eukaryotic (microalgae, yeasts, and fungi), and a variety of marine macroalgae, corals, and other marine life forms. The role that MAAs play as sunscreen compounds to protect against damage by harmful levels of UV radiation is well established. However, evidence is accumulating that MAAs may have additional functions: they may serve as antioxidant molecules scavenging toxic oxygen radicals, they can be accumulated as compatible solutes following salt stress, their formation is induced by desiccation or by thermal stress in certain organisms, they have been suggested to function as an accessory light-harvesting pigment in photosynthesis or as an intracellular nitrogen reservoir, and they are involved in fungal reproduction. Here, the evidence for these additional roles of MAAs as ‘multipurpose’ secondary metabolites is reviewed, with special emphasis on their functions in the microbial world.

Introduction

Mycosporines and mycosporine-like amino acids (MAAs) are low-molecular-weight (generally <400 Da) water-soluble molecules absorbing UV radiation, with the maximum absorbance between 310 and 365 nm. These UV-absorbing pigments were first detected in the 1960s (for a historical overview, see Řezanka et al., 2004), and the first chemical structure of an MAA, in this case a fungal metabolite associated with light-induced sporulation, was published a few years later (Favre-Bonvin et al., 1976; Arpin et al., 1979). It rapidly became clear that these UV-absorbing compounds are extremely widespread in nature. They are accumulated by a wide range of microorganisms exposed to high light intensities. They are found in many cyanobacteria (Garcia-Pichel & Castenholz, 1993) and possibly in some other prokaryotes as well (Arai et al., 1992), in eukaryotic microorganisms (microalgae, yeasts, and fungi), as well as a variety of marine macroalgae, corals, and other marine life forms, including invertebrate and vertebrate animals that derive these compounds from their feed (Karentz et al., 1991). Their presence has also been reported from terrestrial lichens (Büdel et al., 1997; Torres et al., 2004).

Chemically, mycosporines are small (generaly <400 Da) water-soluble molecules, composed of either an aminocyclohexenone or an aminocycloheximine ring, carrying nitrogen or imino alcohol substituents (Carreto et al., 1990; Karentz et al., 1991; Dunlap & Shick, 1998; Karsten et al., 1998; Rozema et al., 2002; Řezanka et al., 2004 see also: http://www.biologie.unierlangen.de/botanik1/eng/maa_database.htm). When substituted with amino acid residues, they are designated MAAs. Their biosynthesis is probably derived from the shikimate pathway for the synthesis of aromatic amino acids, but one's understanding of the enzymatic and genetic basis of their synthesis is still very limited.

Thus far, the chemical structures of over 30 different mycosporines have been elucidated (Bandaranayake, 1998). These include a number of glycosylated mycosporine derivatives. The extracellular glycan sheath of the terrestrial cyanobacterium Nostoc commune contains mycosporines with from three up to eight sugar moieties, and molecular masses up to about 1800 Da (Böhm et al., 1995). The recently characterized mycosporine-glutamicol-glucoside and mycosporine-glutaminol-glucoside are common in terrestrial fungi, aquatic yeasts, and microcolonial ascomycetes (Volkmann et al., 2003; Libkind et al., 2006; Volkmann & Gorbushina, 2006). Figure 1 gives representative examples of the chemical structures of MAAs and other mycosporines.

Chemical structures of representative mycosporines and MAAs.

Mycosporines are widespread in the microbial world. For example, a survey of 152 species of marine microalgae showed that they all contained such UV-absorbing compounds. The highest ratios of UV absorption per chlorophyll were found in surface bloom-forming dinoflagellates, cryptomonads, prymnesiophytes, and raphidophytes (Jeffrey et al., 1999). In many cases, a correlation has been noted between the presence of mycosporines or certain specific representatives of the group and the taxonomic position of the organism investigated, e.g. for cyanobacteria (Karsten & Garcia-Pichel, 1996), pigmented and nonpigmented yeasts (Sommaruga et al., 2004; Libkind et al., 2005), yeast-like and filamentous fungi (Kogej et al., 2006), symbiotic dinoflagellates (Banaszak et al., 2000), and marine macroalgae (Karsten et al., 2005), to the extent that the presence of mycosporines is in many cases less a function of exposure of the organism to high light and UV radiation than of its taxonomic affiliation.

The function that mycosporines play as ‘nature's sunscreen compounds’ has been the subject of a number of review articles (e.g. Bandaranayake, 1998; Dunlap & Shick, 1998; Sinha et al., 1998; Rozema et al., 2002; Shick & Dunlap, 2002). However, in recent years, evidence is accumulating that mycosporines play additional roles. For example, they may serve as antioxidant molecules scavenging toxic oxygen radicals, they function as compatible solutes to protect cells against salt stress, they are involved in protection against desiccation or thermal stress in certain organisms, and they may serve as an intracellular nitrogen reservoir. Following a short overview of the understanding of the function of mycosporines as UV sunscreen compounds, here, the evidence for these additional functions of mycosporines as ‘multipurpose’ secondary metabolites is reviewed. Most information presented below relates to the microbial world. However, as the distribution of mycosporines is not restricted to microorganisms, a few examples of studies on macroorganisms are also given.

MAAs as sunscreen compounds

The review articles cited above give extensive documentation on the occurrence of MAAs and other mycosporines in micro and macroorganisms exposed to high light intensities, their sunscreen function, and their induction following exposure to UV radiation. An in-depth treatment of this aspect of the biology of mycosporines is beyond the scope of this minireview; instead, only a few highlights are presented below.

A correlation between mycosporine content and in situ irradiance levels has been noticed in many locations worldwide and in a wide variety of organisms. As examples of cases in which high light exposure led to MAA accumulation, the dinoflagellate Phaeocystis pouchetii and the diatom Guinardia striata (=Rhizosolenia stolterfothii) in the English channel (Llewellyn & Harbour, 2003), the induction of shinorine in the cyanobacteria Anabaena sp., N. commune, and Scytonema sp. by UV light (Sinha et al., 2001), and the induction of shinorine and porphyra-334 in Antarctic diatoms can be mentioned (Helbling et al., 1996). There is also considerable evidence that indeed the intracellular MAAs provide protection to radiation-sensitive organisms in their natural habitats. This was true for Antarctic phytoplankton blooms (Bracher & Wiencke, 2000), for the red tide dinoflagellate Gymnodinium sanguineum (Neale et al., 1998), as well as for macroalgae (see e.g. Karsten et al., 1998). A study of the competition between two red macroalgae, Chondrus crispus and Mastocarpus stellatus, in the intertidal and upper sublittoral zone of Helgoland (Germany) showed that the much higher MAA content of the latter may confer this alga with a relative advantage in the more light-exposed sites (Bischof et al., 2000).

Mycosporines have also been assigned a photoprotective role in aquatic yeasts, isolated from different freshwater bodies (Libkind et al., 2004). The synthesis of the mycosporine-glutaminol-glucoside (Volkmann et al., 2003) was dramatically stimulated in the presence of photosynthetically active radiation and UV radiation, suggesting a photoprotective function (Libkind et al., 2004). The high concentrations of mycosporine-glutaminol-glucoside observed in the two Rhodotorula species (up to 0.5% of the dry weight) after induction with UV radiation supports the idea that the synthesis of this secondary metabolite is important to obtain protection from UV stress.

The finding that UV light is generally the strongest inducer for the biosynthesis of MAAs agrees well with their function as sunscreen compounds. Special photoreceptors appear to be present to sense the need for MAA synthesis induction, and in most cases wavelengths between 280 and 320 nm are the most effective inducers. This is true for induction of shinorine by the cyanobacteria Chlorogloeopsis PCC 6912 (Portwich & Garcia-Pichel, 1999, 2000) and Anabaena (Rozema et al., 2002; Sinha et al., 2003), for MAA induction in different strains of the cyanobacterial species Nodularia (Sinha et al., 2003), in the marine dinoflagellate Gyrodinium dorsum (Klisch & Häder, 2002), in the Antarctic prymnesiophyte Phaeocystis antarctica (Riegger & Robinson, 1997), in the green alga Prasiola stipata (Rozema et al., 2002), and in the Antarctic pennate diatoms Pseudonitzschia sp. and Fragilariopsis cylindrus (Helbling et al., 1996). In G. dorsum, exposure to short-wavelength UV-B radiation also causes a change in the relative amounts of the different MAAs produced so that shorter wavelengths are preferentially absorbed (Klisch & Häder, 2000). However, visible light is most effective in the induction of MAA synthesis in the Antarctic centric diatoms Thalassiosira sp. and Corethron criophilum (Helbling et al., 1996), and in such cases the action spectrum of MAA induction has its peak in the wavelength range of 370–460 nm (Riegger & Robinson, 1997).

Garcia-Pichel and coworkers have carried out calculations of the possible efficacy of MAAs as sunscreen compounds in planktonic microalgae. MAAs are good sunscreen compounds as they absorb at wavelengths that penetrate relatively deeply in the water; wavelengths below 300 nm are more damaging to DNA and other cellular components, but do not penetrate far in water and are therefore less of a threat to the cell. Cells generally do not contain more than 1% of their dry weight as MAA (Karentz et al., 1991; Garcia-Pichel & Castenholz, 1993). The case of the Euhalothece found in hypersaline environments as discussed below (Oren, 1997) may be a notable exception. The accumulated sunscreens can then be effective only in cells larger than about 100 µm. For cells between 10 and 100 µm, the size of most microalgae, complete protection cannot be achieved, and cells smaller than 10 µm can derive very little protection only from MAAs dissolved in the cytoplasm (Garcia-Pichel & Castenholz, 1993; Garcia-Pichel et al., 1993; Garcia-Pichel, 1994, 1996). When the efficacy of MAAs in solitary planktonic cells or trichomes can thus be doubted, their accumulation in planktonic blooms may increase the UV protection within the community (Sinha et al., 1998).

Theoretically, it is possible to increase the efficacy of MAAs by packaging the pigments around radiation-sensitive sites within the cell (Neale et al., 1998; Shick & Dunlap, 2002). A study of the dinoflagellates Alexandrium tamarense and Heterocapsa triquetra showed that the spectral absorbance of intact cells in the UV range was relatively low in relation to the large amount of total MAA that was released upon freezing and thawing of the cells. A nonhomogeneous distribution of the MAAs within the cell may explain this finding (Laurion et al., 2004). Localized accumulation of MAAs in the extracellular sheath of Nostoc may provide efficient protection (Böhm et al., 1995; Wright et al., 2005). Whether indeed increased concentrations of MAAs may be found in special intracellular compartments in other microorganisms remains to be proven.

MAAs as antioxidant molecules

Some MAAs may protect the cell not only against UV radiation by absorbing the high-energy photons and dissipating the energy as heat but also by scavenging reactive oxygen species such as singlet oxygen, superoxide anions, hydroperoxyl radicals, and hydroxyl radicals.

The first evidence for the antioxidant activity of mycosporine-glycine was published in 2004. Exposure of the scleractinian coral Stylophora pistillata to high temperatures (33°C), causing disruption of the photosynthetic machinery, induces a large increase in superoxide dismutase and catalase activity, while a similar treatment of Platygyra ryukyuensis had little effect on the levels of these enzymes. The difference was correlated with a 20-fold higher content of MAA-glycine in Platygyra. The build-up of oxidative stress caused a profound reduction in the abundance of intracellular mycosporine-glycine, but not of other MAAs, until the compound was completely depleted in Stylophora and strongly reduced in quantity in Platygyra. It was concluded that mycosporine-glycine provides rapid protection against oxidative stress before antioxidant enzymes are induced, and that mycosporine-glycine is thus a biological antioxidant in coral tissue and zooxanthellae (Yakovleva et al., 2004).

The antioxidant activity of mycosporine-glycine was demonstrated in vitro by measuring the chemically initiated, free radical hydroperoxidation of phosphatidylcholine. The presence of molecules with antioxidant activity causes a retardation of the formation of the phosphatidylcholine hydroperoxide. An aqueous extract of the zoanthid (‘rubber coral’) Palythoa tuberculosa, in which mycosporine-glycine is the main MAA, proved to be very effective, and the compound was rapidly degraded by the reactive oxygen species (Dunlap & Yamamoto, 1995). Its presumed precursor 4-deoxygadusol also has a strong antioxidant action. The iminomycosporine-like amino acids shinorine, porphyra-334, palythine, asterina-330, and palythinol were not oxidized (Dunlap & Yamamoto, 1995; Dunlap & Shick, 1998; Shick & Dunlap, 2002).

The efficacy of mycosporine-glycine in protecting biological systems against photodynamic damage by quenching singlet oxygen has been investigated in further detail by examining the effect of 1O2, generated by illumination of eosine Y or methylene blue, on mitochondrial electron transport, lipid peroxidation, hemolysis of erythrocytes, and growth of Escherichia coli. Addition of mycosporine-glycine suppressed singlet oxygen-induced damage in all cases. It was therefore concluded that at least certain MAAs may play a role in protecting marine organisms against sunlight damage, not only by screening energetic UV radiation but also, probably more importantly, by scavenging 1O2 produced by certain endogenous photosensitizers (Suh et al., 2003).

Photosynthetic activity in corals by the symbiotic zooxanthellae can cause local oxygen supersaturation up to 373% air saturation during the daytime (Shashar et al., 1993). Oxygen concentrations as high as 350 µM, equivalent to about 450% oxygen saturation at the ambient salinity and temperature, were measured in a gypsum crust on the bottom of saltern evaporation ponds in Eilat, Israel, populated by a dense community of MAA-rich unicellular cyanobacteria, as described in greater depth in the following section (Canfield et al., 2004). Such high oxygen supersaturation, combined with high light intensities, will probably generate large amounts of reactive oxygen species, and therefore it is quite possible that the MAAs present in such systems may act as antioxidants. However, it should be taken into account that the above-described experiments on the antioxidant activity of MAAs are all based on in vitro models, and that their relevance to in vivo processes remains to be demonstrated.

MAAs and salt stress

The higher the salt concentration in which the cell lives, the higher its intracellular solute concentrations have to be. To provide the necessary osmotic balance, most microorganisms accumulate low-molecular-weight, generally uncharged organic molecules, which serve as so-called ‘osmotic solutes’ or ‘compatible solutes’. The MAAs that accumulate in the cells’ cytoplasm are also small uncharged organic molecules, and thereby they contribute to the osmotic pressure within the cell. The question may therefore be asked to what extent MAAs can contribute to the adaptation of microorganisms to life at high salt concentrations.

MAAs are seldom found in large amounts in cyanobacteria that form blooms in freshwater environments. The recent report of MAAs in a freshwater bloom of Microcystis (Liu et al., 2004) appears to be a rare exception. However, in saline and hypersaline environments, cyanobacteria often contain high concentrations of MAAs. The most extreme case reported is that of the massive development of unicellular cyanobacteria (‘_Euhalothece_’ type) within a benthic gypsum crust in a saltern evaporation pond in Eilat, Israel. The crust is covered by a few centimeters of brine only, so that the cells are exposed to light intensities almost as high as full sunlight. It was estimated that the intracellular concentration of MAAs in these cells reaches values of 100 mM at least or 3% of the cells’ wet weight (Oren, 1997). Although significant, this is still far removed from the concentrations required to balance the salinity of the saltern brines (about 200 g L−1 total dissolved salts), so that additional osmotic solutes — probably glycine betaine — must be present as well. On the other hand, it is probably the highest MAA concentration ever reported; for comparison, values up to 0.8% of the cell dry weight were measured in the cyanobacterium Gloeocapsa (Garcia-Pichel & Castenholz, 1993). Cyanobacterial cells cultured from the gypsum crust contained MAAs: mycosporine-2-glycine with the maximum absorbance at 331 nm (Kedar et al., 2002) and a novel compound with the maximum absorbance at 362 nm (‘euhalothece-362’), recently identified as 2-(E)-3-(E)-2,3-dihydroxyprop-1-enylimino-mycosporine-alanine (Volkmann et al., 2006; see also Fig. 1). The possible involvement of these MAAs in osmotic regulation was demonstrated in experiments in which the salinity of the medium was reduced by dilution with fresh water: the MAAs were rapidly excreted to the medium in amounts proportional to the degree of dilution (Oren, 1997).

Similar phenomena were described in studies of Chlorogloeopsis strain PCC 6912, which tolerates salt up to 70% seawater salinity, and reportedly produces shinorine and MAA-glycine. Also, here, the amount of MAAs present in the cell can provide no more than 5% of the total intracellular osmolyte concentration needed for osmotic balance, and glucose and trehalose have been identified as the main compatible solutes. Also, here, UV stress induces synthesis of MAAs, but their steady-state levels are also positively correlated with environmental salinity. In an experimental setup in which cells were incubated in the dark with sucrose as an energy source, visible or UV light was not required during the biosynthesis of MAAs. The biosynthesis of mycosporine-glycine was salinity-controlled and synergistically enhanced by UV-B, whereas accumulation of shinorine was largely controlled by the levels of UV-B radiation. As in the case of the Eilat gypsum crust, hypoosmotic shock resulted in leakage of MAA (preferentially of mycosporine-glycine) to the medium (Portwich & Garcia-Pichel, 1999, 2000). MAAs may be involved in the adaptation of sea ice algae to osmotic changes when variations in ambient temperature cause large changes in sea ice brine salinity (Arrigo & Thomas, 2004).

There is also some indication for an osmotic function of mycosporines in halotolerant fungi. Cladosporium sphaerospermum, Cladosporium cladosporioides, the halophilic black yeasts Phaeotheca triangularis and Hortaea werneckii, and the halotolerant Aureobasidium pullulans (all order Dothideales) contain mycosporine-glutamicol-glucoside as their major mycosporine, while only black yeast also contains a smaller amount of mycosporine-glutaminol-glucoside. These fungi had a much higher content of mycosporine-glutaminol-glucoside when grown in 10% salt than in a salt-free medium. This substance may thus act as a supplementary compatible solute in some extremophilic black yeasts exposed to saline growth medium. The levels of mycosporine-glutamicol-glucoside were somewhat depressed in high-salt-grown cells. Not all black yeasts show this behavior: increased salt concentrations did not induce significantly different mycosporine levels in Trimmatostroma salinum (Kogej et al., 2006). Hortaea werneckii grown at increasing NaCl levels up to 10%, coinciding with its optimal growth conditions, had increased levels of mycosporines, while their concentration decreased at higher salinities (unpublished data). Surprisingly, the halophilic fungal genus Wallemia, including the most halophilic species known to date, Wallemia ichthyophaga, does not produce mycosporines or any other UV-absorbing compounds. The genus Wallemia belongs to the phylogenetically old classes Wallemiomycetes (order Wallemiales), distantly related to the phylum Basidiomycota (Zalar et al., 2005).

MAAs and desiccation stress

There are many reports on the occurrence of high concentrations of mycosporines in microorganisms exposed to drought stress, such as cyanobacteria (Tirkey & Adhikary, 2005; Wright et al., 2005) and rock-dwelling microcolonial fungi (Gorbushina et al., 2003). The cyanobacterium N. commune, which, in its natural habitat, is subjected to simultaneous stress of desiccation, UV radiation, and oxidation, possesses a thick extracellular matrix in which glycosylated MAAs are embedded. A detailed analysis has been carried out of the three-dimensional structure of this matrix and the function of a 33.6 kDa protein WspA in its formation and the mode of binding of mycosporines and scytonemin, another extracellular cyanobacterial UV-absorbing compound (Wright et al., 2005).

In most cases, these organisms are exposed to high radiation and other forms of stress as well, and controlled experiments to elucidate the role of desiccation in the induction of mycosporines are scarce, and so are experiments to test whether the presence of mycosporines contributes to the drought resistance of the organisms. Tirkey & Adhikary (2005), in their study of cyanobacteria in biological soil crusts in India (dominated by filamentous sheath-forming species including Lyngbya, Plectonema, and Scytonema), stated that MAA synthesis is stimulated by the combination of desiccation and irradiation to which the organisms are exposed. However, no experimental evidence was presented to support this assumption. Experiments with the unicellular cyanobacterium Gloeocapsa sp. (an organism that produces MAA with the maximum absorbance at 326 nm but does not make scytonemin) showed that under conditions of desiccation, with physiological photoprotective and repair mechanisms inoperative, MAA-containing cells were only slightly more resistant to high UV radiation than control cells, this in contrast to wet cells that were effectively protected by the MAAs. Active photoprotective and/or repair mechanisms are thus essential for survival under high UV radiation, and MAAs alone do not provide sufficient protection (Garcia-Pichel et al., 1993).

Colonies of black melanized fungi (Sarcinomyces, Coniosporium, Phaeotheca) growing on desert rock surfaces, where they are exposed to desiccation, UV radiation, and nutrient scarcity, contain high concentrations of mycosporine-glutaminol-glucoside. The presence of this compound may be related to the survival potential and longevity of the vegetative hyphae of these fungi (Gorbushina et al., 2003).

MAAs and thermal stress

There are a few reports on the induction of MAA formation by high-temperature stress. MAA content in the soft corals Lobophytum compactum and Sinularia flexibilis in the Great Barrier Reef was upregulated under themal stress (increase in water temperature to 32°C), and their concentrations were further enhanced during simultaneous exposure to UV (Michalek-Wagner, 2001; Shick & Dunlap, 2002). However, in contrast to UV and salt stress, increased temperature stress did not induce MAA formation in the cyanobacterium Chlorogloeopsis PCC 6912, nor did cold shock, nutrient limitation, or photooxidative stress (Portwich & Garcia-Pichel, 1999). Owing to the high incidence of MAA-producing microorganisms in cold aquatic environments, it is quite possible that multipurpose MAAs act as compatible solutes under freezing conditions; however, a thorough survey of the potential cold-induction of mycosporines has not yet been performed.

MAAs as accessory pigments in photosynthesis?

In an early study (Sivalingam et al., 1976), it was suggested that MAAs may increase photosynthetic efficiency. At the time, it was noted that MAAs are fluorescent compounds. Following excitation in the UV-A range, emission of fluorescence at wavelengths close to the absorbance of the Soret band of chlorophyll a was observed, so that theoretically energy transfer from MAA to chlorophyll could be possible. However, MAAs are only weakly fluorescent, if at all, and MAAs are generally most abundant in high-light environments, in which light energy is not the limiting factor for photosynthesis. So far, the early claim that MAAs may serve as accessory pigments in photosynthesis has never since been substantiated.

MAAs as intracellular nitrogen storage

MAAs are nitrogenous compounds, the most abundant types containing two nitrogen atoms per molecule. Accordingly, it has been proposed that MAAs may also serve as an intracellular nitrogen storage (Korbee Peinado et al., 2004; Korbee et al., 2006). In a study on the red macroalga Porphyra columbina from the Patagonian coast, combined stimulation of MAA formation (porphyra-334 and shinorine) by ammonium ions and UV radiation was found. In this study, discs of the alga were incubated in the presence of 0, 50, and 300 µM NH4+; at the highest concentration, significantly more MAA was made than at the lower concentrations (Korbee Peinado et al., 2004).

If indeed MAAs may serve as intracellular nitrogen storage, mechanisms should be present to enable mobilization of the nitrogen whenever other suitable forms of nitrogen are in short supply. However, nothing is known about the possible intracellular degradation of MAAs and release of their nitrogen atoms, so that there is little experimental evidence as yet to support the idea that MAAs can indeed be accumulated as nitrogen storage molecules.

Mycosporines and fungal reproduction

Light, and more specifically UV light, is often a requirement for the formation of reproductive organs in fungi. It is sensed by light-induced pigments that specifically absorb at 240 and 310 nm. Colonies grown in the dark that do not contain such UV-absorbing pigments do not sporulate. The majority of these compounds have absorption maxima at 310 nm and they were therefore called ‘P310’. The first P-310 substance was isolated from the sporophores of the basidiomycete Stereum hirsutum (Leach, 1965), and its structure was elucidated as mycosporine-serinol (Favre-Bonvin et al., 1976).

The designation ‘P310’ actually refers to three substances that absorb differently at wavelengths shorter than the 310 nm maximum, and they also differ in their sporogenic activity. Later, several other mycosporines have been isolated from different fungi. Nowadays, it is known that mycosporines are widespread among the fungal classes Zygomycetes, Deuteromycetes, Ascomycetes, and Basidiomycetes, but they are absent from some distinct phylogenetic lineages, such as the Wallemiales (Zalar et al., 2005), Agaricales (Arpin et al., 1979), Sporidiobolales, and Cystofilobasidiales (Libkind et al., 2004). In terrestrial lichens involving fungal-cyanobacterial symbioses, only oxo-carbonyl mycosporines were found (Řezanka et al., 2004), probably produced by the fungal partner. Ecologically, mycosporine-producing fungi are terrestrial or aquatic; they can be saprophytic or phytopathogenic, and some are opportunistic human pathogens. They live as filamentous fungi on different surfaces or as free-living yeasts in fresh water or in seawater. They are found in temperate zones as well as in extreme environments, including polar regions, glacial ice, hot and cold deserts, and hypersaline waters.

Near UV radiation-induced sporogenic mycosporines (P-310 metabolites) were detected in the mycelia of several terrestrial fungal genera, but they were absent from nonsporulating colonies grown in darkness, as seen in the ascomycetous fungus Pyronema omphalodes. Pyronema omphalodes cultures contained different mycosporines during the formation of fruiting bodies, and nor-mycosporine-glutamine was found in the primordia. Its concentration decreased as the culture ripened, while an increase in mycosporine-glutaminol-glucoside was observed in the mature ascoma, particularly in the ascospores (Bernillon et al., 1984). The biogenetic chemical modification of nor-mycosporine probably occurs during ascospore formation, conveying at the same time a progressive chemical stability of the final compound.

In the phytopathogenic anamorphic Ascochyta fabae, another mycosporine is produced during reproduction and accumulated in spores (Bandaranayake, 1998). Since these initial discoveries, mycosporines or their biochemical precursors have been related to sporulating mycelia and were considered as biochemical markers for reproductive states of fungi or as reproduction markers (Gorbushina et al., 2003).

As a general conclusion, mycosporines or related compounds were always detected in ascomata and the sporulating hymenium of UV-induced sporulating mycelia and sclerotia, but not in nondifferentiated mycelia, rhizomorphs, nonsporulating mycelia, or sporulating mycelia grown in darkness. Thus, the synthesis and occurrence of mycosporines appeared to be linked strictly to the sporulation process (Leach, 1965; Bandaranayake, 1998; Řezanka et al., 2004). The highest concentrations were found in the conidiogenous thallus (both macro- and micro-conidia), intermediate in the perithecial thallus, and the lowest in the vegetative mycelium (Bernillon et al., 1984; Bandaranayake, 1998). The quantitative variation of mycosporines during thallus development and their accumulation inside spores indicates translocation from sites of synthesis into reproductive cells (Bandaranayake, 1998; Gorbushina et al., 2003).

The rate of mycosporine production varied with the wavelength of irradiation, period of irradiation, light intensity, and nutritional conditions of the culture. Changes in irradiance induce a rapid response at high light intensity and short wavelengths. However, light is required only to initiate the process of mycosporine production, as their synthesis continues in the dark as well as in the light. The exception are those fungi in which light is an absolute requirement for sporogenesis (Bandaranayake, 1998; Řezanka et al., 2004).

The in vitro experiments demonstrated a relationship between the nature and the content of free sterols and different degrees of sexual morphogenesis, induced either by light or by mycosporines. These experiments suggest that mycosporines may be a biochemical transmitter of light energy, required for the morphogenetic transformation. This function appears to be regulated through light-induced and mycosporine-mediated changes in sterol metabolism (Bandaranayake, 1998; Gorbushina et al., 2003).

Mycosporines cannot fundamentally be considered as direct photoproducts, but appear to be connected to a type of metabolism that occurs at a very low level during mycelial growth. Their production increases during the reproductive morphogenesis, and they can thus mediate changes in the cell membranes and the structure of the cell wall (Bandaranayake, 1998).

Mycosporines are present in the mucilage that surrounds conidia of some fungi, but there was no evidence of their presence inside the conidia (Young & Patterson, 1982). At least in some fungi, mycosporines protect conidia from solar radiation during atmospheric dispersal and prevent untimely germination (Leite & Nicholson, 1992), in both ways prolonging their survival. Mycosporines have also been located in the extracellular matrix and in the outer cell wall layers of microcolonial fungi, in which they mediate a whole range of intracellular changes. The ecological group of extremophilic microcolonial fungi constitutively synthesizes considerable amounts of mycosporines (Gorbushina et al., 2003). These fungi are characterized by slow-growing restricted colonies, which grow by mitotic cell divisions. Cells within the microcolonies grow by enlargement and intracellular septation (de Hoog & Hermanides-Nijhof, 1977). Microcolonies avoid the complex process of sexual reproduction and/or sporulation by combining the properties of a growing colony and of a sclerotium, a vegetative survival structure. Thus, microcolonial fungi cells more closely resemble spores than growing cells. It was hypothesized that microcolonies have their origin in aborted sexual fruiting bodies (Gorbushina, 2003).

Final comments

MAAs and other mycosporines are true ‘multipurpose’ secondary metabolites, which have many additional functions in the cell beyond their well-known UV sunscreen role. The function of mycosporines in osmotic stress in certain cyanobacteria and yeasts (Oren, 1997; Portwich & Garcia-Pichel, 1999; Kogej et al., 2006), the role of at least certain MAAs as scavengers of free radicals of oxygen (Dunlap & Yamamoto, 1995; Suh et al., 2003), and their possible role in desiccation resistance (Gorbushina et al., 2003; Tirkey & Adhikary, 2005; Wright et al., 2005) are just a few examples of other functions that these versatile molecules may have in different biological systems. Little is known as yet about the relative importance of these and other ‘alternative’ functions of MAAs and about the effects of different forms of stress on their synthesis.

There is little doubt that the sunscreen role is the most important function that MAAs and other mycosporines play in nature. This is clearly shown by the abundance of such compounds in organisms exposed to high levels of radiation, and by the regulation of the synthesis of mycosporines in many organisms according to the ambient intensity of UV light. However, a full understanding of their function should also take the less known aspects into account.

Acknowledgements

The authors would like to acknowledge Dr Zdravko Podlesek for the drawing of the MAA structures, presented in Fig. 1.

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Author notes

Editor: Rustam Aminov